I knew a professor who was sued by a student over her grade on an exam. Going through that made him keep the originals of each student exam and only return photocopies so that he would have your original work if it ever came to that in court.
I had a professor brag about a legal battle where he got GitHub to take down a student's submission for the class project (which was of course the same every year) because it contained his "copyrighted material" (read: a mediocre, incomplete MIPS emulator and hacked-together toolchain). Some professors are just not very nice people about these things.
Can you explain why professors and students would be adversaries?